This invention relates to a method and apparatus for improved flow control, particularly by providing improved control of a metering valve. More particularly, the invention relates to a method and apparatus for improving the accuracy, repeatability and speed of control in a flow control system.
It is often desired to control the flow of a fluid. An important area of application for flow control is in controlling mass-flow. For example, in semiconductor fabrication it is routinely important to introduce controlled quantities of liquids or gases for depositing precision films, or precise concentrations of dopant materials. Metering valves are provided for this purpose and typically employ an aperture and an actuating rod which translates toward or away from the aperture, the actuating rod having a plunger which blocks the aperture to a variable and controllable degree. The actuating rod, which is typically formed of a ferro-magnetic material, is typically spring-biased in one direction, e.g, wherein the valve is either open or closed, and controllably urged in the other direction by a magnetic field. The selected magnetic field is created by passing a corresponding selected electrical input current through a coil which is coaxially disposed around the rod. Thence, the selected electrical input current correlates with a desired linear position of the rod, or mass-flow "setting."
The setting of a typical metering valve is controlled by a control system which receives a set-point value of flow, measures the actual flow and, by providing an appropriate electrical input to the metering valve, instructs the metering valve to move the actuating rod to make the measured flow equal the set-point value. Especially in relatively inexpensive flow metering systems, the control system typically assumes that a given electrical input will produce the correct setting in the metering valve. However, all metering valves have mechanical and electrical tolerance errors that prevent achieving arbitrarily accurate or repeatable settings in response to given electrical inputs. This error typically includes a bias component and a random component, both of which may be expected to vary with the setting. Although it is the task of the control system to correct for this error, the demand placed upon the control system generally increases its response time and decreases its stability.
Goettling et al., U.S. Pat. No. 5,197,508 ("Goettling") proposes a valve apparatus and method for controlling fluid flow that provides a closed-loop control circuit for the fluid valve itself, for eliminating the aforementioned error. Goettling measures the position of a permanent magnet affixed to a valve actuating rod with a field sensor, such as a Hall effect sensor, to produce a feed-back signal transmitted to a proportional solenoid for controlling the position of the rod.
A disadvantage of Goettling, however, is that it requires a separate permanent magnet to be installed on a valve rod, resulting in additional mechanical complexity and cost. Another disadvantage of the use of the permanent magnet of Goettling is that the valve employing it must be specially manufactured. The magnet cannot be added later, and the scheme will not work with a standard valve.
Still another disadvantage with employing the permanent magnet of Goettling is in electronic complexity. The magnetic field of the permanent magnet will bias the field sensor. For example, a Hall effect sensor has a transfer characteristic that proportionally relates input magnetic field and output voltage. A permanent magnet may bias the sensor into a region that is not linear, requiring a complex electronic compensation. And even if it does not, the simple, proportional relationship is lost and some additional complexity is required to un-bias the sensor.
It is sometimes particularly desirable to employ stainless steel, plastic or other materials having relatively great physio-chemical resistance in a metering valve because, often, the fluids passed through these valves are caustic. As it is sometimes not possible to seal the internal workings of the valve completely from these fluids, limitations in the choice of materials, such as any requirement to employ a permanent magnet, may be additionally undesirable.
Accordingly, there is a need for a method and apparatus for improved flow control that provides for responsive, accurate, repeatable and stable control of a fluid flow metering valve at a minimum cost.